Variable Message Coding Protocols for Encoding Auxiliary Data in Media Signals

ABSTRACT

Variable message coding protocols enable greater flexibility in encoding auxiliary data in media signals. One such protocol employs a version identifier that indicates the type of coding used to process an auxiliary data message before it is embedded in a host media signal. This version identifier specifies the type of error robustness coding applied to a variable message. The error robustness coding may be varied to alter the message payload capacity for different versions of auxiliary data embedding and reading systems. Another protocol uses control symbols to specify the format and variable length of the variable message.

RELATED APPLICATION DATA

This application is a continuation of application Ser. No. 10/193,719,filed Jul. 10, 2002 (now U.S. Pat. No. 7,412,072), which is acontinuation in part of application Ser. No. 10/020,519, filed Dec. 14,2001 (published as US20020159614), which claims the benefit ofprovisional application 60/256,627, filed Dec. 18, 2000. ApplicationSer. No. 10/193,719 is also a continuation in part of application Ser.No. 10/137,124, filed May 1, 2002, which claims the benefit ofprovisional application 60/288,272, filed May 1, 2001. Application Ser.No. 10/193,719 is also a continuation in part of application Ser. No.09/186,962, filed Nov. 5, 1998 (now U.S. Pat. No. 7,171,016), which is acontinuation of application Ser. No. 08/649,419, filed May 16, 1996 (nowU.S. Pat. No. 5,862,260). These patents and applications are herebyincorporated by reference.

TECHNICAL FIELD

The invention relates to digital watermarking, steganography, andspecifically to message coding protocols used in conjunction withdigital watermarking and steganographic encoding/decoding methods.

BACKGROUND AND SUMMARY

Digital watermarking is a process for modifying physical or electronicmedia signals to embed a hidden machine-readable code into the media.The media signal may be modified such that the embedded code isimperceptible or nearly imperceptible to the user, yet may be detectedthrough an automated detection process. Most commonly, digitalwatermarking is applied to media signals such as images, audio signals,and video signals. However, it may also be applied to other types ofmedia objects, including documents (e.g., through line, word orcharacter shifting), software, multi-dimensional graphics models, andsurface textures of objects. Steganography is related field of studypertaining to encoding and decoding of hidden auxiliary data signals,such that the auxiliary data is not discernable by a human.

Digital watermarking systems typically have two primary components: anencoder that embeds the watermark in a host media signal, and a decoderthat detects and reads the embedded watermark from a signal suspected ofcontaining a watermark (a suspect signal). The encoder embeds awatermark by subtly altering the host media signal. The readingcomponent analyzes a suspect signal to detect whether a watermark ispresent. In applications where the watermark encodes information, thereader extracts this information from the detected watermark.

Several particular watermarking and steganographic techniques have beendeveloped. The reader is presumed to be familiar with the literature inthis field. Particular techniques for embedding and detecting auxiliarymessages in media signals are detailed in the assignee's U.S. Pat. Nos.6,614,914, and 6,122,403, which are hereby incorporated by reference.

One practical challenge in the deployment of digital watermarkingsystems is the potential lack of flexibility in changing aspects of thedigital watermark system once its deployed. As system and applicationrequirements change, there is sometimes a desire to change aspects ofthe digital watermark message coding protocol. For example, one mightwant to change the format, syntax, semantics and length of the messagepayload in the digital watermark. The syntax used in the protocol caninclude the types and sizes of message fields, as well as the symbolcoding alphabet (e.g., use of binary or M-ary symbols, etc.) Thesemantics used in the protocol refer to the meaning of the messageelements in the message payload (e.g., what the elements are interpretedto mean). While such changes may not alter the fundamental data hidingor extraction function, they present a practical difficulty because thedeployed digital watermark readers may be rendered obsolete if theprotocol is changed.

One potential solution is to upgrade the readers deployed in the field.However, this presents technical challenges, such as whether the readersare accessible and/or re-programmable to receive and facilitateupgrades.

The invention provides variable message protocol methods for digitalwatermarking. One aspect of the invention is a message protocol methodfor digital watermarking. This method forms a fixed message protocolportion having a fixed length and identifying a version of a variableprotocol portion. The method also forms a variable message protocolportion having variable error robustness message coding format. Theversion indicates the error robustness coding format of the variableprotocol portion. The fixed and variable message protocol portions arethen embedded into a host media signal such that the message issubstantially imperceptible in the host media signal.

Another aspect of the invention is a method for decoding a digitalwatermark having fixed and variable protocol message portions. Themethod extracts a hidden message code embedded in a host media signal byevaluating the host media signal to compute the hidden message codehaving fixed and variable message protocol portions. It performs errorrobustness decoding of the fixed protocol portion of the extractedmessage code to produce one or more message symbols representing aversion identifier. Next, it interprets the version identifier toascertain a version of variable protocol used to embed the variableprotocol portion. Finally, it applies an error robustness decodingmethod of the version to decode message symbols of the variable messageprotocol portion.

Another aspect of the invention is a message protocol method forsteganographically encoding a variable message into a media signal. Thismethod forms a control message portion including at least one symbolthat identifies the format of the variable message. It also forms avariable message according to the format. The format indicates avariable length of the variable message portion. The method produces amedia signal with the variable message steganographically encoded in itsuch that the variable message is not discernable by a human but isreadable by an automated reader. For example where the media signal isan image, the a human viewer is not able to read the variable messageencoded in that image because symbols in the variable message arearranged so as not to be interpretable without knowledge of the encodingformat.

Further features will become apparent with reference to the followingdetailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an extensible message protocol methodfor digital watermark embedding.

FIG. 2 is a diagram illustrating a method of extracting a digitalwatermark message from a host media signal that has been embedded usingthe method of FIG. 1.

FIGS. 3A and 3B show examples of bit cells used in one form of digitalwatermark embedding.

FIG. 4 shows a hierarchical arrangement of signature blocks, sub-blocks,and bit cells used in one implementation of a digital watermark messageprotocol.

DETAILED DESCRIPTION

FIG. 1 is a diagram illustrating a message protocol method for digitalwatermark embedding. The protocol in this context refers to how themessage is prepared for digital watermark embedding into a host mediasignal. One attribute specified by the message protocol is the errorrobustness coding that is applied to the message. Error robustnesscoding includes operations on the message that make it more robust toerrors that undermine its complete and accurate recovery in potentiallydistorted version of the watermarked host media signal. Specific formsof error robustness coding include repetition of one or more parts ofthe message, and error correction coding of one or more parts of themessage.

Another aspect of the message protocol is the length of the messagepayload. The message payload is a variable part of the message. It canbe variable in both content (e.g., the values of the individual messagesymbols in the payload are variable), and length (e.g., the number ofsymbols is variable). This message payload enables the digital watermarksystem to convey unique information per watermarked item, such as anitem ID, a transaction ID, a variable ASCII character message, etc.

A related aspect of the message protocol is the syntax and semanticmeaning of the message elements. As the length of the payload isincreased or decreased, the fields within that payload may change, aswell as the semantic meaning of the fields. For example, the first Nbinary symbols may represent a unique ID, while the next M bitsrepresent a source ID or hash of the object in which the information isembedded. As N and M change and other fields are added or deleted, thesyntax and semantic meaning of symbols in the payload change.

Yet another aspect of the protocol is the extent to which it facilitatesdigital watermarking systems that have different message protocols, yetare backward and/or forward compatible with each other. Backwardcompatibility refers to the case where new versions of the digitalwatermark reader are able to read messages using the most recentlyreleased protocol version, as well as messages in every prior protocolversion. Forward compatibility refers to the case where a currentversion of the digital watermark reader is able to read messagescompatible with subsequently released protocol versions. Furtherexamples illustrating this aspect of the protocol follow later.

The method illustrated in FIG. 1 operates with many different forms ofdigital watermark embedding and detecting operations. In other words,regardless of how the host media signal is modified to embed the resultof the message protocol (referred to as the intermediate signal), themessage protocol method is widely applicable.

The method of FIG. 1 also operates on different host media signal typesand formats. For the sake of illustration, we will use examples of stillimage watermark embedding that are extendable to other media types, suchas motion images (e.g., video) and audio. The method is implemented insoftware and operates on blocks of the host media signal of a fixedsize. These blocks are typically much smaller than the overall size ofthe host signal, and as such, are tiled or otherwise repeated throughoutthe host signal to provide an additional layer of robustness beyond therobustness coding within each block.

Since the blocks are of fixed size in our example implementation, thereare trade-offs between the length of the variable message payload andthe extent of redundancy that may be employed to map that variablemessage payload into the host media signal of fixed size.

As shown in FIG. 1, the message 100 has a fixed protocol portion 102 anda variable protocol portion 104. The fixed protocol portion includes afixed message part 106, and a variable message part 108. Each of theparts of the fixed protocol portion have a fixed length, and employ afixed error robustness coding method. The fixed message part includes afixed set of known message symbols that serve as a test for falsepositives (e.g., provide a check to ensure a valid digital watermark ispresent).

The variable part carries a version identifier 108. This versionidentifier may carry version parameters, such as an error correctiontype identifier, a repetition indicator, an error detect indicator or anindex that refers to the type of error correction, error detection,and/or repetition applied in the variable protocol portion 104. Thevariable part of the fixed protocol varies so as to indicate the versionof the variable protocol used in processing the variable protocolportion.

The variable protocol portion 104 includes a variable payload part 110and an error detect part 112. As noted earlier, the payload has avariable number of symbols (X) as specified by the version. The protocolemploys a form of error detection, such as a certain type and length ofCyclic Redundancy Check symbols. The variable message protocol portion,therefore, includes a number of error detect symbols (Y).

The message protocol method generates a message code signal 114 byperforming error robustness coding on the fixed and variable protocolportions. In the case of the fixed protocol, the method uses a fixederror correction coding method 116 followed by fixed repetition 118 ofthe resulting message a predetermined number of times (n). While thediagram shows error correction followed by repetition coding, the errorrobustness coding of the fixed portion may include error correctionand/or repetition coding. Examples of error correction coding includeblock codes (e.g., BCH, Reed Solomon, etc.), convolution codes, turbocodes or combinations thereof.

The version parameters 120 in the illustrated example specify thepayload and error detection part lengths, and number of repetitions ofthe variable portion or individual parts of the variable portion. Theymay also specify the type of error correction coding to be applied, suchas block codes, convolution codes, concatenated codes, etc. As explainedfurther below, some forms of error correction, such as convolutioncodes, perform error correction in a manner that depends on subsequentsymbols in the message symbol string. As such, symbols at the end of thestring are error correction decoded with less confidence because thereare fewer or no symbols following them. This attribute of errorcorrection coding schemes that have “memory” can be mitigated byrepeating parts of the message symbol string that are more susceptibleto errors due to the lack of memory than other parts of the messagesymbol string. As noted, this typically leads to repetition of the tailof the string more than the beginning of the string.

According to the version parameters 120, the protocol method applies aselected error correction coding 122 to the symbols of the variableportion 104, and then applies repetition coding 124 to one or more partsof the error correction coded symbols.

The protocol method then appends 126 the robustness coded fixed andvariable portions to form a message code signal 114.

For added security in some applications, the method transforms (128) themessage code signal with a secret key. This transformation may include avector XOR or matrix multiplication of a key 130, such as pseudorandomnumber that is sufficiently independent from other like key numbers,with the message code signal. The key may be a seed number to apseudorandom sequence generator, an index to a look up table thatproduces a vector or matrix, or a vector/matrix, etc. The key serves thefunction of making the digital watermark un-readable to anyone exceptthose having the proper key. The use of this key enables the digitalwatermarking protocol to be used for several entities wishing toprivately embed and read their own digital watermarks, through the useof their own keys.

The result of the transformation by the key 130 is the secure messagecode 132. Our example implementation applies an additionaltransformation to the secure message code before embedding it into thehost media signal block. In particular, a mapping function 134 mapselements of the secure message code vector to elements of the hostsignal block. The elements of the host signal block may becharacteristics of individual samples (luminance of pixels or frequencycoefficients), or characteristics of groups of samples (statisticalfeatures). The carrier signal function 136 transforms the message codeelements as a function of corresponding elements of a carrier signal.One such example is spread spectrum modulation of the secure messagecode with a carrier signal. The carrier signal may have attributes thatincrease robustness of the watermark (message spreading and scatteringas an anti-jamming mechanism), and facilitate detection and geometricsynchronization (e.g., autocorrelation properties). The result oftransformation by the carrier and mapping functions 138 is anintermediate signal. A digital watermark embedder 140 then modifiescharacteristics of elements of the host media signal block according tothe elements of the intermediate signal to hide the intermediate signalin the host media signal block. There are a wide variety of suchembedding methods that may be employed, including those discussed in thedocuments incorporated by reference. Where perceptual artifacts are aconcern, human perceptual modeling may be employed to reduce theperceptibility of artifacts caused by modifying the host media signalblock according to the intermediate signal.

FIG. 2 is a diagram illustrating a method of extracting a digitalwatermark message from a host media signal that has been embedded usingthe method of FIG. 1. This method is implemented in a softwareimplementation of a digital watermark reader. The reader extractsestimates of values for the intermediate signal from the host, using areader 150 compatible with the embedder 140 of FIG. 1. This process maybe performed after filtering, synchronizing and generating blocks of thehost media signal. In our implementation, the reader 150 extractsestimates of the intermediate signal elements. It then uses the mappingfunction 152 and carrier signal 154 to convert elements of theintermediate signal embedded in each host media signal block to softestimates of the secure message code. These elements are soft estimatesderived from aggregating elements from the intermediate signal estimatefor each corresponding element of the secure message code 156 accordingto the mapping and carrier functions. In particular, each soft messagecode element represents a value between S, and −S, where S represents aninteger corresponding to binary symbol 1, and −S represents the negativeinteger corresponding to binary symbol 0.

Next, the reader transforms (158) the secure message code estimate withthe key 160. This operation reverses the key transformation 128 appliedto the message code in the embedder of FIG. 1. The result is a messagecode signal estimate, which includes the fixed and variable messageprotocol portions. The reader extracts these portions (164, 166) andproceeds to apply the fixed protocol to decode the error robustnesscoding of the fixed protocol portion. This entails accumulation 168 ofthe repeated message symbols, followed by error correction decoding 170.

The result of the error correction decoding includes a set of fixedsymbols (the false positive symbols) 172, and the version identifier174. The reader compares the extracted fixed symbols with the actualfixed symbols 176, and if there is a match 178, then the versionidentifier is deemed to be accurate. The reader interprets the versionidentifier to get the version parameters 180, such as the errorcorrection coding type for the variable protocol, the repetitionparameters, the structure of the variable protocol portion, etc. Theversion parameters may be carried within the version identifier directlyor may be accessed via a look-up operation, using the version identifieras an index.

With this version information, the reader proceeds to decode the errorrobustness coding of the variable protocol portion. This decodingentails, for example, accumulation 182 of the repeated symbols to undothe repetition coding, along with error correction decoding 184according to the version information. The result of the decodingincludes the payload 186 and error detection symbols 188. The readerapplies the error detection method to the payload and compares 190 withthe error detection symbols to confirm the accuracy of the payloadinformation.

This protocol portion enables the watermarking system to be backward andforward compatible. It is backward compatible because each new versionof watermark detector may be programmed to read digital watermarksembedding according to the current version and every prior version ofthe protocol. It can be forward compatible too by establishing versionidentifiers and corresponding protocols that will be used in futureversions of the system. This enables watermark detectors deployedinitially to read the current version of the protocol, as well as futureversions of the protocol as identified in the version identifier. At thetime of embedding a particular media signal, a digital watermarkembedder embeds a version identifier of the protocol used to embed thevariable protocol portion. At the time of reading the digital watermark,a reader extracts the version identifier to determine the protocol ofthe variable protocol portion, and then reads the message payloadcarried in the variable protocol portion.

Another embodiment of a digital watermarking protocol is described inU.S. Pat. No. 5,862,260, which is incorporated by reference. In thisprotocol, the digital watermark message includes a control messageprotocol portion and a variable message protocol portion. The controlmessage includes control symbols indicating the format and length of thevariable message protocol portion. The control message protocol and thevariable message protocol include symbols that are mapped to locationswithin a block of the host signal called a “signature” block. As thelength of the variable message portion increases, the redundancy of thecontrol message portion decreases.

U.S. Pat. No. 5,862,260 describes a variety of digital watermarkembedding methods. One such class of methods for images and videoincrements or decrements the values of individual pixels, or of groupsof pixels (bumps), to reflect encoding of an auxiliary data signalcombined with a pseudo random noise signal. One variation of thisapproach is to embed the auxiliary data—without pseudo randomization—bypatterned groups of pixels, termed “bit cells.”

Referring to FIGS. 3A and 3B, two illustrative 2×2 bit cells are shown.FIG. 3A is used to represent a “0” bit of the auxiliary data, while FIG.3B is used to represent a “1” bit. In operation, the pixels of theunderlying image are tweaked up or down in accordance with the +/−valuesof the bit cells to represent one of these two bit values. The magnitudeof the tweaking at any given pixel, bit cell or region of the image canbe a function of many factors, including human perceptibility modeling,non-linear embedding operations, etc. as detailed in U.S. Pat. No.5,862,260. In this case, it is the sign of the tweaking that defines thecharacteristic pattern. In decoding, the relative biases of the encodedpixels are examined using techniques described above to identify, foreach corresponding region of the encoded image, which of the twopatterns is represented.

While the auxiliary data is not explicitly randomized in thisembodiment, it will be recognized that the bit cell patterns may beviewed as a “designed” carrier signal.

The substitution of a pseudo random noise carrier with a “designed”information carrier affords an advantage: the bit cell patterningmanifests itself in Fourier space. Thus, the bit cell patterning can actlike the subliminal digital graticules discussed in U.S. Pat. No.5,862,260 to help register a suspect image to remove scale/rotationerrors. By changing the size of the bit cell, and the pattern therein,the location of the energy thereby produced in the spatial transformdomain can be tailored to optimize independence from typical imageryenergy and facilitate detection.

While the foregoing discussion contemplates that the auxiliary data isencoded directly—without randomization by a PRN signal, in otherembodiments, randomization can of course be used.

FIG. 4 illustrates an example of a digital watermarking protocol havinga message control portion and a variable portion. While this protocol isillustrated using an image, it applies to other media types and digitalwatermark embedding/reading systems.

Referring to FIG. 4, an image 1202 includes a plurality of tiled“signature blocks” 1204. (Partial signature blocks may be present at theimage edges.) Each signature block 1204 includes an 8×8 array ofsub-blocks 1206. Each sub-block 1206 includes an 8×8 array of bit cells1208. Each bit cell comprises a 2×2 array of “bumps” 1210. Each bump1210, in turn, comprises a square grouping of 16 individual pixels 1212.

The individual pixels 1212 are the smallest quanta of image data. Inthis arrangement, however, pixel values are not, individually, the datacarrying elements. Instead, this role is served by bit cells 1208 (i.e.2×2 arrays of bumps 1210). In particular, the bumps comprising the bitscells are encoded to assume one of the two patterns shown in FIG. 3. Asnoted earlier, the pattern shown in FIG. 3A represents a “0” bit, whilethe pattern shown in FIG. 3B represents a “1” bit. Each bit cell 1208(64 image pixels) thus represents a single bit of the embedded data.Each sub-block 1206 includes 64 bit cells, and thus conveys 64 bits ofembedded data.

The nature of the image changes effected by the encoding follows thetechniques set forth in U.S. Pat. No. 5,862,260 under the heading MOREON PERCEPTUALLY ADAPTIVE SIGNING.

In the illustrated embodiment, the embedded data includes two parts:control bits and message bits. The 16 bit cells 1208A in the center ofeach sub-block 1206 serve to convey 16 control bits. The surrounding 48bit cells 1208B serve to convey 48 message bits. This 64-bit chunk ofdata is encoded in each of the sub-blocks 1206, and is repeated 64 timesin each signature block 1204.

A digression: in addition to encoding of the image to redundantly embedthe 64 control/message bits therein, the values of individual pixels areadditionally adjusted to effect encoding of subliminal graticulesthrough the image. In this embodiment, the graticules discussed inconjunction with FIG. 29A in U.S. Pat. No. 5,862,260 are used, resultingin an imperceptible texturing of the image. When the image is to bedecoded, the image is transformed into the spatial domain, aFourier-Mellin technique is applied to match the graticule energy pointswith their expected positions, and the processed data is theninverse-transformed, providing a registered image ready for decoding(see U.S. Pat. No. 5,862,260). The sequence of first tweaking the imageto effect encoding of the subliminal graticules, or first tweaking theimage to effect encoding of the embedded data, is not believed to becritical. As presently practiced, the local gain factors (discussed inU.S. Pat. No. 5,862,260) are computed; then the data is encoded; thenthe subliminal graticule encoding is performed. Both of these encodingsteps make use of the local gain factors.

Returning to the data format, once the encoded image has been thusregistered, the locations of the control bits in sub-block 1206 areknown. The image is then analyzed, in the aggregate (i.e. consideringthe “northwestern-most” sub-block 1206 from each signature block 1204),to determine the value of control bit #1 (represented in sub-block 1206by bit cell 1208Aa). If this value is determined (e.g. by statisticaltechniques of the sort detailed above) to be a “1,” this indicates thatthe format of the embedded data conforms to the standard detailedherein. According to this standard, control bit #2 (represented by bitcells 1208Ab) is a flag indicating whether the image is copyrighted.Control bit #3 (represented by bit cells 1208Ac) is a flag indicatingwhether the image is unsuitable for viewing by children. Certain of theremaining bits are used for error detection/correction purposes.

The 48 message bits of each sub block 1206 can be put to any use; theyare not specified in this format. One possible use is to define anumeric “owner” field and a numeric “image/item” field (e.g. 24 bitseach).

If this data format is used, each sub-block 1206 contains the entirecontrol/message data, so same is repeated 64 times within each signatureblock of the image.

If control bit #1 is not a “1,” then the format of the embedded datadoes not conform to the above described standard. In this case, thereading software analyzes the image data to determine the value ofcontrol bit #4. If this bit is set (i.e. equal to “1”), this signifiesan embedded ASCII message. The reading software then examines controlbits #5 and #6 to determine the length of the embedded ASCII message.

If control bits #5 and #6 both are “0,” this indicates the ASCII messageis 6 characters in length. In this case, the 48 bit cells 1208Bsurrounding the control bits 1208A are interpreted as six ASCIIcharacters (8 bits each). Again, each sub-block 1206 contains the entirecontrol/message data, so same is repeated 64 times within each signatureblock 1204 of the image.

If control bit #5 is “0” and control bit #6 is “1,” this indicates theembedded ASCII message is 14 characters in length. In this case, the 48bit cells 1208B surrounding the control bits 1208A are interpreted asthe first six ASCII characters. The 64 bit cells 1208 of theimmediately-adjoining sub-block 1220 are interpreted as the final eightASCII characters.

Note that in this arrangement, the bit-cells 1208 in the center ofsub-block 1220 are not interpreted as control bits. Instead, the entiresub-block serves to convey additional message bits. In this case thereis just one group of control bits for two sub-blocks.

Also note than in this arrangement, pairs of sub-blocks 1206 containsthe entire control/message data, so same is repeated 32 times withineach signature block 1204 of the image.

Likewise if control bit #5 is “1” and control bit #6 is “0.” Thisindicates the embedded ASCII message is 30 characters in length. In thiscase, 2×2 arrays of sub-blocks are used for each representation of thedata. The 48 bit cells 1208B surrounding control bits 1208A areinterpreted as the first six ASCII characters. The 64 bit cells of eachof adjoining block 1220 are interpreted as representing the next 8additional characters. The 64 bits cells of sub-block 1222 areinterpreted as representing the next 8 characters. And the 64 bit cellsof sub-block 1224 are interpreted as representing the final 8characters. In this case, there is just one group of control bits forfour sub-blocks. And the control/message data is repeated 16 timeswithin each signature block 1204 of the image.

If control bits #5 and #6 are both “1's”, this indicates an ASCIImessage of programmable length. In this case, the reading softwareexamines the first 16 bit cells 1208B surrounding the control bits.Instead of interpreting these bit cells as message bits, they areinterpreted as additional control bits (the opposite of the casedescribed above, where bit cells normally used to represent control bitsrepresented message bits instead). In particular, the reading softwareinterprets these 16 bits as representing, in binary, the length of theASCII message. An algorithm is then applied to this data (matching asimilar algorithm used during the encoding process) to establish acorresponding tiling pattern (i.e. to specify which sub-blocks conveywhich bits of the ASCII message, and which convey control bits.)

In this programmable-length ASCII message case, control bits aredesirably repeated several times within a single representation of themessage so that, e.g., there is one set of control bits forapproximately every 24 ASCII characters. To increase packing efficiency,the tiling algorithm can allocate (divide) a sub-block so that some ofits bit-cells are used for a first representation of the message, andothers are used for another representation of the message.

Reference was earlier made to beginning the decoding of the registeredimage by considering the “northwestern-most” sub-block 1206 in eachsignature block 1204. This bears elaboration.

Depending on the data format used, some of the sub-blocks 1206 in eachsignature block 1204 may not include control bits. Accordingly, thedecoding software desirably determines the data format by firstexamining the “northwestern-most” sub-block 1206 in each signature block1204; the 16 bits cells in the centers of these sub-blocks will reliablyrepresent control bits. Based on the value(s) of one or more of thesebits (e.g. the Digimarc Beta Data Format bit), the decoding software canidentify all other locations throughout each signature block 1204 wherethe control bits are also encoded (e.g. at the center of each of the 64sub-blocks 1206 comprising a signature block 1204), and can use thelarger statistical base of data thereby provided to extract theremaining control bits from the image (and to confirm, if desired, theearlier control bit(s) determination). After all control bits havethereby been discerned, the decoding software determines (from thecontrol bits) the mapping of message bits to bit cells throughout theimage.

To reduce the likelihood of visual artifacts, the numbering of bit cellswithin sub-blocks is alternated in a checkerboard-like fashion. That is,the “northwestern-most” bit cell in the “northwestern-most” sub-block isnumbered “0.” Numbering increases left to right, and successivelythrough the rows, up to bit cell 63. Each sub-block diametricallyadjoining one of its corners (i.e. sub-block 1224) has the same orderingof bit cells. But sub-blocks adjoining its edges (i.e. sub-blocks 1220and 1222) have the opposite numbering. That is, the “northwestern-most”bit cell in sub-blocks 1220 and 1222 is numbered “63.” Numberingdecreases left to right, and successively through the rows, down to 0.Likewise throughout each signature block 1204.

In a variant of this format, a pair of sub-blocks is used for eachrepresentation of the data, providing 128 bit cells. The center 16 bitcells 1208 in the first sub-block 1206 are used to represent controlbits. The 48 remaining bit cells in that sub-block, together with all 64bit cells 1208 in the adjoining sub-block 1220, are used to provide a112-bit message field. Likewise for every pair of sub-blocks throughouteach signature block 1204. In such an arrangement, each signature block1204 thus includes 32 complete representations of the encoded data (asopposed to 64 representations in the earlier-described standard). Thisadditional length allows encoding of longer data strings, such as anumeric IP address (e.g., URL).

Obviously, numerous alternative data formats can be designed. Theparticular format used can be indicated to the decoding software byvalues of one or more control bits in the encoded image.

From the foregoing examples, there are a variety of ways to implementvariable message protocols. In one approach having a fixed and variablemessage protocol, the fixed protocol portion is mapped to a fixed partof the host signal, and does not vary in length. In another approach,the number of locations in the host signal used to represent the messagecontrol portion decrease as the length of the variable messageincreases. The control portion may remain fixed, as in the first case,even if the variable message varies in length, by varying therepetition/error correction coding applied to the variable messageportion.

Use of Variable Repetition with Error Correction Coding

U.S. application Ser. No. 10/020,519 by Bradley and Brunk explained thatthe tail of a convolutionally coded message is more error prone than therest of the message. One way to make the tail more robust to errors isapply a block error correction code, such as a BCH or other block errorcorrection code, to the tail portion of the message. In this approach,the encoder applies block error correction coding to all, or just thetail of a message sequence, and then follows with convolutional codingof the resulting message sequence. The decoder then reverses thisprocess, effectively using the block error correction to correct errorsin the tail of the message.

U.S. Application No. 60/288,272 by Sharma and Decker discusses the useof repetition and error correction coding. One way to compensate for theerrors in the tail of a convolutionally coded message is to userepetition coding, where symbols of the convolutionally coded messageare repeated, and specifically repeated in a variable fashion. Themessage symbols of the error correction coded message that are moreprone to error, such as the tail symbols of the message in aconvolutionally coded message, are repeated more than symbols at thebeginning or middle of the message.

These approaches extend generally to error correction coding schemeswith memory, where lack of memory at a part of the message makes thatpart more error prone. In particular, selective block coding or variablerepetition coding of the error prone part improves the error robustnessof the digital watermark message. Block error correction codes, unlikeconvolutional codes, do not have memory. Memory refers to the attributeof the coding method where subsequent symbols are used to correct errorsin previous symbols. Variable repetition coding may be performed onindividual error correction coded symbols, or blocks of such symbols.Preferably, more error prone symbols are repeated more than less errorprone, error correction coded symbols.

Another way to address the error prone tail part of a convolutionallycoded message is to use tail biting codes, where the tail of the codedmessage loops around to the head or start of the coded message. Suchtail biting codes may suffer from being too computationally complexrelative to the improvement in error robustness that they can provide.

Returning to the specific approach of using variable repetition, we haveexperimented with a number of variable repetition assignments for errorcorrection coded symbols of digital watermark messages. A programmaticprocess generates the assignments from a curve that represents therepetition per symbol position over a sequence of message symbols in adigital watermark message from the start of the message to its end or“tail.” Our experiments show that a variable repetition curveapproximating a tan hyperbolic function, comprising constant repetitionrate per symbol followed by an increasing repetition rate per symbol,and ending in a constant repetition rate, provides improved errorrobustness relative to the use of a constant repetition rate throughoutthe error correction encoded message.

Further experiments show that a variable repetition curve, starting witha constant repetition rate for the beginning of the message, andconcluding with a linear increase in the repetition rate at the middleto end of the message also provides improved error robustness.

These curves may be approximated with a staircase shaped curvecomprising segments of constant repetition rates at different levels ofrepletion. In some implementations, these stair case approximations areconvenient because they facilitate the use of scrambling/encryption ofthe output of the repetition coder, and also facilitate decoding of adigital watermark message with fixed and variable protocol portions asdescribed above.

The effect of this approach is to set a variable signal to noise for theerror correction coded symbols through variable repetition rates ofthose symbols. Relative to constant repetition rate coding of errorcorrection coded symbols, this approach achieves a lower effective errorrate for the same signal to noise ratio of the digital watermark messagesignal.

Automated and/or programmatic methods may be used to find optimizedvariable repetition curves for a given digital watermark message model.Our experience shows that the errors introduced by the digitalwatermarking channel on the error correction coded message areapproximated by white guassian noise. As such, our programmaticprocesses model the channel, and use general parameters definingcharacteristics of the curve, to compute the repetition rate per errorcorrection coded symbol that achieves preferred error robustness.

The first step in formulating a repetition rate per symbol curveinvolves choosing an appropriate model. It is not a requirement tochoose a parametric model, but it is a convenience. The principle basisfor consideration of a model is that it is monotonically increasing.Further, it should allow flexibility in tuning the initial point ofrepetition increase as well as the rate of increase, which may or maynot be constant. We, for example, have found that both the hyperbolictangent and the piece-wise linear constant model behave satisfactorily.

Once a model is chosen it remains to vary its parameters until the bestbehavior in terms of minimum error rate is found. Specifically, if onecan model the noise characteristics of the digital watermark message atthe input to the convolutional decoder, it is desirable to run manysimulations with pseudo-randomly generated noise in order to determinehow the model and corresponding choice of parameters behave. If a slightperturbation in the model parameters produces a better simulation effect(e.g., lower error rate), we continue to adjust the parameters in thedirection of the perturbation. One programmatic process for convergingon an optimized result is a gradient-descent procedure. The modelparameters are adjusted using such a procedure, according toperturbation and simulation re-evaluation, until a minimum in the errorrate is achieved. In order to avoid problems with local minima on theoptimization surface and/or simulation noise, one may wish to performthe search using several different initial parameter configurations. Itshould be noted that for all choices of models and correspondingparameters, the total number of repetitions should remain fixed. Inother words, the area under the repetition curve is constant.

Extensions

The above concepts of protocols with variable robustness coding may beextended to optimize auxiliary data coding applications, includingdigital watermarking. Generally stated, the approach described in theprevious section uses variable robustness coding to reduce the errorrate in more error prone parts of a steganographic message. One specificform of variable robustness coding is variable repetition coding of moreerror prone parts of an error correction coded message.

One variation of this approach is to analyze a model of the channeland/or the host media signal that is communicated through that channelto determine locations within the steganographic code (e.g., embeddinglocations of a digital watermark) that are likely to be more errorprone. In these locations, the steganographic encoding process uses amore robust message coding scheme than in other locations that are lesserror prone. One specific example is to subdivide the host media signal,such as an image, video frame or audio file into blocks, such as thecontiguous tiles described above. Then, the embedder measures theexpected error rate for each block, and applies an amount of errorrobustness coding to the steganographic code mapped to that blockcorresponding to the expected error rate. Higher error rate blocks havea greater amount of robustness coding, such as more repetition permessage symbol. For example, for fixed sized tiles, the error robustnesscoding increases, resulting in fewer message symbols in the block, butat a higher error robustness level.

The measurement of expected error rate can be modeled based on a modelof the channel and/or model of the host signal. For example, the hostsignal may have certain properties that make the steganographic codeembedded in it more error prone for a particular channel. For example,an image that has less variance or energy in a block may be more errorprone for a distortion channel that includes printing, scanning, and/orcompression. As such, a measure of the variance in the block provides anindicator of the error rate, and thus, an indicator of the type of errorrobustness coding that need by applied to reduce the error rate. Theerror robustness, such as the extent of repetition coding or strength ofthe error correction code is selected to correspond to the desired errorrate for the block.

One challenge in supporting such variable robustness coding withinblocks of a host signal is the extent to which the auxiliary datadecoder (e.g., digital watermark reader) is able to interpret variablerobustness coding. This can be addressed by using a message protocolwith fixed and variable protocol portions, where the fixed portion ineach block specifies the type of error robustness coding used for thatblock. Alternatively, if the embedder uses a robust measure ofachievable capacity for a given error rate, it is possible to determinethe amount and/or type of robustness coding that was used at the encoderby observing the data at the auxiliary data decoder. In this way, thedecoder can exploit what it knows about the channel, namely, thereceived host signal carrying the auxiliary data (e.g., an imagecarrying a digital watermark) and supposed processing noise, in the samefashion that it was exploited at the embedder of the auxiliary data. Inparticular, if the measure of the expected error rate is likely to bethe same at the embedder and the decoder, even after distortion by thechannel and the embedding of the auxiliary data, then the decoder cansimply re-compute the expected error rate at the receiver, and use thismeasure to determine the type of error robustness coding that has beenapplied. In another words, a part of the auxiliary data need not beallocated to identifying the type of error robustness coding if thedecoder can derive it from the received signal, the channel, and/orother information available to it.

Concluding Remarks

Having described and illustrated the principles of the technology withreference to specific implementations, it will be recognized that thetechnology can be implemented in many other, different, forms. Thevariable message coding protocols may be used in digital watermarkingapplications where digital watermarks are embedded by imperceptiblymodifying a host media signal. They may also be used in steganographicapplications where message are hidden in media signals, such as images(including graphical symbols, background textures, halftone images,etc.) or text. The embedding or encoding of the message according to thevariable protocols may, in some cases create visible structures orartifacts in which the message is not discernable by a human, yet isreadable by an automated reader with knowledge of the protocol,including any keys used to scramble the message.

To provide a comprehensive disclosure without unduly lengthening thespecification, applicants incorporate by reference the patents andpatent applications referenced above.

The methods, processes, and systems described above may be implementedin hardware, software or a combination of hardware and software. Forexample, the auxiliary data encoding processes may be implemented in aprogrammable computer or a special purpose digital circuit. Similarly,auxiliary data decoding may be implemented in software, firmware,hardware, or combinations of software, firmware and hardware. Themethods and processes described above may be implemented in programsexecuted from a system's memory (a computer readable medium, such as anelectronic, optical or magnetic storage device).

The particular combinations of elements and features in theabove-detailed embodiments are exemplary only; the interchanging andsubstitution of these teachings with other teachings in this and theincorporated-by-reference patents/applications are also contemplated.

1. A message protocol method comprising: forming a fixed messageprotocol portion and identifying a version of a variable protocolportion; forming the variable message protocol portion having variableerror robustness message coding format, wherein the version indicatesthe error robustness coding format of the variable protocol portion; anddigitally watermarking message symbols of the fixed and variable messageprotocol portions into a host media signal by altering features thereof,such that the message is substantially imperceptible in the host mediasignal.
 2. The method of claim 1 wherein the error robustness codingformat includes error correction coding.
 3. The method of claim 2wherein the error robustness coding format includes repetition coding ofan error correction coded message.
 4. The method of claim 3 wherein therepetition coding repeats certain error correction coded symbolsdifferently than other error correction coded symbols.
 5. The method ofclaim 1 wherein the fixed message portion includes fixed message symbolsalong with one or more variable symbols indicating the version of thevariable protocol portion.
 6. The method of claim 1 wherein the versionindicates one or more parameters of the error robustness message codingformat.
 7. The method of claim 6 wherein the one or more parametersinclude a parameter of an error correction coding method.
 8. The methodof claim 6 wherein the version indicates a number of symbols in thevariable protocol portion before the symbols are error robustness coded.9. The method of claim 6 wherein the one or more parameters include arepetition indicator specifying a number of repetitions of a message inthe variable protocol portion.
 10. A computer readable medium on whichis stored instructions for performing the method of claim
 1. 11. Amethod of decoding a digital watermark having fixed and variableprotocol message portions, the method comprising: extracting a hiddenmessage code embedded in a host media signal by slight changes tofeatures thereof, by evaluating the host media signal to compute thehidden message code having the fixed and variable message protocolportions; performing error robustness decoding of the fixed protocolportion of the extracted message code to produce one or more messagesymbols representing a version identifier; interpreting the versionidentifier to ascertain a version of variable protocol used to embed thevariable protocol portion; and applying an error robustness decodingmethod of the version to decode message symbols of the variable messageprotocol portion.
 12. The method of claim 11 wherein the errorrobustness decoding method includes error correction decoding specifiedby the version.
 13. The method of claim 11 wherein the error robustnessdecoding includes a particular format of repetition coding specified bythe version.
 14. A message protocol method for steganographicallyencoding a variable message into a media signal, the method comprising:forming a control message portion including at least one symbol thatidentifies the format of the variable message; forming a variablemessage according to the format, wherein the format indicates a variablelength of the variable message portion; and producing a media signalwith the variable message steganographically encoded in the media signalthrough altered features thereof, such that the variable message is notdiscernable by a human but is readable by an automated reader.
 15. Themethod of claim 14 wherein the format indicates how the variable messageis repeated within the media signal.
 16. The method of claim 15 whereinthe format indicates how the variable message is repeated within a blockof the media signal, wherein the block has a fixed size.
 17. The methodof claim 16 wherein the variable message is encoded into the fixed sizeblocks, which are tiled across the media signal.
 18. The method of claim14 wherein the media signal comprises an image.
 19. The method of claim14 wherein the variable message carries information related to the mediasignal in which it is encoded.
 20. A computer readable medium on whichis stored instructions for performing the method of claim 14.